1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a method and apparatus for driving a liquid crystal display. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for improving a picture quality in the liquid crystal display which is operated under different driving frequencies.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) controls a light transmittance of each liquid crystal cell in accordance with a video signal, thereby displaying a picture. An active matrix LCD including a switching device for each liquid crystal cell is suitable for displaying a moving picture. The active matrix LCD uses a thin film transistor (TFT) as a switching device.
The LCD has a disadvantage in that it has a slow response time due to inherent characteristics of a liquid crystal, such as a viscosity and an elasticity, etc. Such characteristics can be explained by using the following equations (1) and (2):τr∝γd2/Δε|Va2−VF2|  (1)where τr represents a rising time when a voltage is applied to a liquid crystal, Va is an applied voltage, VF represents a Freederick transition voltage at which liquid crystal molecules begin to perform an inclined motion, d is a cell gap of the liquid crystal cells, and γ represents a rotational viscosity of the liquid crystal molecules.τf∝γd2/K  (2)where τf represents a falling time at which a liquid crystal is returned into the initial position by an elastic restoring force after a voltage applied to the liquid crystal was turned off, and K is an inherent elastic constant of a liquid crystal.
A twisted nematic (TN) mode liquid crystal has a different response time due to physical characteristics of the liquid crystal and a cell gap, etc. Typically, the TN mode liquid crystal has a rising time of 20 to 80 ms and a falling time of 20 to 30 ms. Since such a liquid crystal has a response time longer than one frame interval (i.e., 16.67 ms in the case of NTSC system) of a moving picture, a voltage charged in the liquid crystal cell is progressed into the next frame prior to arriving at a target voltage. Thus, due to a motion-blurring phenomenon, a moving picture is blurred out on the screen.
Referring to FIG. 1, the conventional LCD cannot express desired color and brightness. Upon implementation of a moving picture, a display brightness BL fails to arrive at a target brightness corresponding to a change of the video data VD from one level to another level due to its slow response time. Accordingly, a motion-blurring phenomenon appears from the moving picture and a display quality is deteriorated in the LCD due to a reduction in a contrast ratio.
In order to overcome such a slow response time of the LCD, U.S. Pat. No. 5,495,265 and PCT International Publication No. WO99/05567 have suggested to modulate data in accordance with a difference in the data by using a look-up table (hereinafter referred to as high-speed driving scheme). This high-speed driving scheme allows data to be modulated by a principle as shown in FIG. 2.
Referring to FIG. 2, a conventional high-speed driving scheme modulates input data VD and applies the modulated data MVD to the liquid crystal cell, thereby obtaining a desired brightness MBL. In the high-speed driving scheme, |Va2−VF2| is increased from the above equation (1) on the basis of a difference of the data so that a desired brightness can be obtained in response to a brightness value of the input data within one frame interval, thereby rapidly reducing a response time of the liquid crystal. Accordingly, the LCD employing such a high-speed driving scheme compensates for a slow response time of the liquid crystal by modulating a data value in order to alleviate a motion-blurring phenomenon in a moving picture, thereby displaying a picture at desired color and brightness.
In other words, the high-speed driving scheme compares most significant bits MSB of the previous frame Fn−1 with those of the current frame Fn. If there is a change in the most significant bits, the corresponding modulated data Mdata are selected from the look-up table to modulate the data as shown in FIG. 3. The high-speed driving scheme modulates only several most significant bits to reduce a memory size upon implementation of hardware equipment. A high-speed driving apparatus implemented in this manner is as shown in FIG. 4.
Referring to FIG. 4, a conventional high-speed driving apparatus includes a frame memory 43 connected to a most significant bit bus line 42 and a look-up table 44 commonly connected to the most significant bit bus line 32 and an output terminal of the frame memory 43.
The frame memory 43 stores most significant bit data MSB during one frame interval and supplies the stored data to the look-up table 44. Herein, the most significant bit data MSB may be the most significant 4 bits of the 8-bit source data RGB.
The look-up table 44 compares most significant bits MSB of a current frame Fn inputted from the most significant bit line 42 with those of the previous frame Fn−1 inputted from the frame memory 43 as shown in Table 1 or Table 2, and selects the corresponding modulated data Mdata. The modulated data Mdata are added least significant bits LSB from a least significant bit bus line 41.
TABLE 101234567891011121314150013467910111214151515151510124579101112131415151515201235789101213141515151530123568910111214141515154001246791011121314151515500023578911121314151515600013468910111314151515700012457810111214141515800012356891112131415159000123467910121314151510000012457810111314151511000002356791112141515120000013457810121315151300000123468101113141514000000123579111314151500000001246911131415
TABLE 2016324864809611212814416017619220822424000324864809611214416019220822424024024024016016486480961121281601922082242402402402403200326480961121281601922082242402402402404800164880961121281601762082242402402402406400164864961121281441761922082242402402408000163248801121281441761922082242402402409600163248649612814416019220822424024024011200163248648011214416017620822424024024012800163248648096128160176192224240240240144001632486480961121441761922082242402401600016324864809611212816019220822424024017600163248648096112128144176208224240240192001632486480961121281441601922242402402080016324848648096112128160176208240240224001632484864809611212814417619222424024000016324848648096112128144176208240
In the above tables, a left column is for a data voltage VDn−1 of the previous frame Fn−1 while an uppermost row is for a data voltage VDn of the current frame Fn. Table 1 is a look-up table information in which the most significant bits (i.e., 20, 21, 22 and 23) are expressed by the decimal number format. Table 2 is a look-up table information in which weighting values (i.e., 24, 25, 26 and 27) of the most significant 4 bits are applied to 8-bit data.
However, the conventional high-speed driving scheme still has a problem. Since it has been studied on the assumption that a driving frequency of the source data is fixed like a television, it was difficult to apply the scheme to a frequency-variable display device receiving different driving frequencies like a computer monitor. More specifically, in the conventional high-speed driving scheme, a voltage level of the modulated data Mdata is fixed to a specific frequency (e.g., 60 Hz) and a response time (i.e., 16.7 ms) of the liquid crystal fixed in accordance with the specific frequency. On the other hand, a computer monitor is manufactured so that its driving frequency can be changed in the range of 50 to 80 Hz. Therefore, in order to apply the conventional high-speed driving scheme to such a computer monitor, the modulated data Mdata established in the conventional high-speed driving scheme should be modified based on a driving frequency. This is because a voltage charged in a liquid crystal should be modified based on a driving frequency to adjust a response time of the liquid crystal. As a result, when the modulated data Mdata established based on a fixed driving frequency are applied to a monitor displaying a picture at a driving frequency different from the specific frequency, a picture is more deteriorated.